Pultrusion is a technique in which longitudinally continuous fibrous structures are used to pull a resin through a die so that the resin sets and produces a rigid part downstream of the die to which the pulling force is applied.
Originally the longitudinal fibres consisted of simply longitudinal in the extending rovings and the parts were of a relatively simple cross section such as rods, T-bars and the like. However developments have been introduced to reduce the wall thickness of the parts so that complex cross sections including hollow cross sections could be manufactured. In order to achieve the necessary strength of the parts, it was necessary to introduce transverse fibers to provide strength in the transverse direction. Such transverse fibers are conventionally applied using a mat of a woven or non woven material. In many cases the fibers in the mat are generally random so that the number of fibers extending in the transverse direction is relatively small. One major problem with the mat is that it is relatively expensive and can be very expensive so that it is more than double per pound of the cost of the simple conventional rovings. One direction of development has been to provide improved mats which apply more of the fibers in the transverse direction thus allowing the mat to be of reduced thickness to provide the required strength or toughness in the finished part. An example of a mat tailored for pultrusion is shown in published International application PCT/WO78529A1 published Dec. 28, 2000 and assigned to Pella.
An alternative approach to the expense of the mat is to attempt to attach to the longitudinal rovings some transverse fibers which are simply chopped roving material. One example of an arrangement of this type is shown in U.S. Pat. No. 5,324,377 of the present inventor Davies. This method of pultrusion attempted to attach transverse fibers to the outside of a body of longitudinal rovings so as to be carried through the die with the rovings. This method has to date not achieved commercial success.
In order to minimize material costs, commercial pultrusion is normally carried out using polyester as the resin which is a simple thermo-set resin material so that it can be applied to the fibers from a bath and is thermo-set within the heated die. However polyester is a linear polymer which is thus not cross-linked so that it is relatively brittle so that it tends to crack when bending forces are applied to the part. This cracking is reduced by providing in the reinforcing fibers a mat material at the surface of the wall of the part so that the fibers are held in place by the transverse fibers, This avoids or reduces the tendency of the longitudinal fibers to break through the surface of the part on bending of the part. Until now, therefore, polyester resin parts have required that the mat be applied on the outside surface. Thus in a hollow cross section, a mat is applied on the surface of each wall of the part so that there is a mat on the outside and also a mat on the inside surface with the normal longitudinal fibers or rovings being placed in between the two mats. This construction significantly increases the cost of the part in that the mat is relatively expensive and two mats are required. Also the provision of two mats increases the thickness of the part and thus again increases material costs.
Non-linear resins, which may be cross-linked or amorphous such as epoxy resins, have been available for some years and are used in pultrusion; but these are significantly more expensive than polyester and thus have not achieved significant market penetration in conventional simple parts for which pultrusion is ideally suited.
Recently, improved non-linear resin materials, particularly those based on urethane, have become available which are still more expensive than polyester but are not as expensive as other cross-linked resins such as epoxy. These cross-linked urethane resins could simply be used in replacement for the conventional polyester which might lead to increased strength where required or desirable. However and in most cases the resins are of increased cost so it is necessary to consider the possibility of otherwise reducing material costs to maintain a competitive economic product while at least matching the strength of the competitive polyester product.
Mats for reinforcing pultruded parts are provided to add structural strength and in order to provide the required or expected amount of strength have a weight of fibers greater than 0.5 ounces per square foot and generally 0.75 to 1.0 ounces per square foot
Veils, which are used to provide surface characteristics and not to provide any structural strength are lighter, generally less than 0.5 ounces per square foot and typically of the order of 0.1 ounces per square foot. Conventional veils are used outside rovings or outside mats at the surface to provide an improved surface appearance or to retain the stiffer glass fibers within the resin to prevent fiber “bloom” or projecting fibers which can act as slivers. This latter requirement to prevent slivers is particularly important in tool handles or similar products. The retention of fibers to prevent weathering or bloom is particularly important in fenestration or similar products. Veils are well known and well used, when required for the part concerned, by persons skilled in this art and are not intended to form part of and are not considered as part of the fiber reinforcement.